Material characterisation for simulation of press hardening

University dissertation from Luleå : Luleå tekniska universitet

Abstract: The growing effort to reduce vehicle weight and improve passive safety in the automotive industry has drastically increased the demand for ultra high strength steel components. There exist several production methods for such components, where the press hardening technique (hot stamping) is one of the most successful in producing complex components from boron steel. In order to accurately perform numerical Finite Element (FE) simulations of the actual thermo-mechanical forming, it is crucial to use correct material data and models. This work is focusing on two main aspects of the material characterisation as follows. The first is the flow stress of the austenite at elevated temperatures and different strain rates, relevant for the process, which is crucial for correctly predicting the strains in the component and the forming force. During a press hardening cycle, the actual forming is performed at high temperatures and the steel is in the austenitic state. The second, the austenite decomposition into daughter products such as ferrite, pearlite, bainite or martensite is a function of the thermal and mechanical history. To find the mechanical response (flow stress) for the austenite, a method based on multiple overlapping continuous cooling and compression tests (MOCCCT) in combination with inverse modelling has been developed. A validation test (in combination with the compression tests) shows good agreement with the simulated forming force, indicating that the estimated flow stress as a function of temperature, strain and strain rate is accurate in the actual application. The austenite decomposition model is developed and integrated as a material subroutine into the FE-code LS-DYNA. The model is based on the combined nucleation and growth rate equations proposed by Kirkaldy. A separate test to simulate different cooling histories along a boron alloyed steel sheet has been conducted.

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